1. Field of the Invention
The present invention relates in general to an interconnect structure for providing signal paths between test equipment and contact pads on a semiconductor wafer, and in particular to a wafer interconnect structure employing a closed-grid bus to distribute signals to several integrated circuit devices under test.
2. Description of Related Art
In many applications microstrip or stripline traces convey a logic signal from a single source to many nodes on a printed circuit board (PCB). For example traces on a motherboard commonly distribute data, address and control lines of computer buses to sockets holding daughterboards, or traces on a PCB may distribute a clock signal to several synchronously operating integrated circuits (ICs) mounted on the PCB.
A typical integrated circuit (IC) tester includes a test head structure containing a set of tester channels. Each tester channel may either transmit a test signal to an IC input/output (I/O) terminal or monitor a IC output signal appearing at an IC I/O terminal to determine whether the IC is behaving as expected in response to its input signals. When ICs are tested while still in the form of die on a semiconductor wafer, a “prober” typically holds a wafer adjacent to the test head and provides a set of probes contacting I/O pads of one or more ICs. An interconnect structure is provided to link the tester channels to the probes. A typical interconnect structure includes a circuit board having upper and lower surfaces containing contact pads. A separate pogo pin extending from each tester channel contacts a separate one of the upper surface contact pads. An “interposer” mounted between the circuit board and the prober includes spring contacts linking the contact pads on the lower surface of the circuit board to the probes. Traces on various layers of the circuit board, and vias interconnecting those traces provide appropriate signal paths between the upper and lower contact pads.
Normally the interconnect structure links each tester channel to only a single I/O pad of a single IC. However in some cases, for example when a tester channel is supplying power to ICs under test, the interconnect structure may connect a single channel to more than power supply pad.
Since there are usually more I/O pads than available tester channels, an IC tester can test only a portion of the ICs on the wafer at any one time. Thus the “prober” holding the wafer must reposition the wafer under the probes several times so that all ICs can be tested. It would be advantageous if all ICs on a wafer could be contacted and tested concurrently without having to reposition the wafer.
One way to reduce the number of tester channels needed to do this is to concurrently connect the same tester channel to corresponding I/O pads of a-large number of ICs on the wafer. For example an IC tester tests a random access memory (RAM) by writing data into each RAM address, reading it back out, and determining whether the data read out of the RAM matches the data written into it. When the tester has a sufficient number of channels to separately access I/O pads of more than one RAM, it can independently test several RAMs concurrently.
However it is also possible for a tester to concurrently test several RAMs without requiring so many tester channels by connecting the data and address I/O pads of several RAMs in parallel to the same set of tester channels while connecting the control I/O pads of the RAMs to separate tester channels. This arrangement enables the tester to concurrently write access several RAMs while allowing it to consecutively read access each RAM. The arrangement therefore reduces the number of write cycles needed to test the RAMs, substantially reduces the number of channels needed to concurrently test all of the RAMs on the wafer, and eliminates the need to position the wafer under the interconnect structure more than once. Thus instead of providing a set of signal paths, each connecting a single tester channel to a single IC pad, an interconnect structure for a wafer-level tester could provide a set of buses, each providing a path from a single tester channel to a large number probes accessing IC pads.
When such buses are formed by traces on a printed circuit board (PCB) each bus should make efficient use of PCB area since many buses must share a relatively small amount of PCB area above each IC. Also each bus should deliver the signal to ICs with as little variation in edge timing as possible and with as little distortion as possible.
Logic signals have been commonly distributed to many nodes on a PCB using stripline or microstrip traces in a “daisy-chain”, or “star” or “stubbed” bus configurations. FIG. 1 illustrates a conventional daisy-chain bus configuration wherein traces 10 on a PCB 12 connect a set of bus nodes 14 in series to route an incoming signal (IN) to each bus node. The daisy-chain configuration makes efficient use of PCB space. However since each IN signal edge must travel a relatively large distance between the first and last nodes 13 and 15, and must charge IC input capacitance as it arrives at several intermediate nodes 14, the time difference between detection of IN signal edges by ICs connected to nodes 13 and 15 can be relatively large. A long-daisy chain bus can also severely distort the signal wave front as it passes from node-to-node; the last node on the bus will see substantially slower rise time than the first. Such wave front distortion tends to increase the variation in signal path delay between the first and last nodes 13 and 15 on the daisy-chain bus.
A daisy-chain bus is also intolerant of open-circuit faults; an open circuit fault anywhere on the daisy-chain bus will prevent the signal from reaching any node beyond the fault. Since an interconnect structure for a wafer-level tester would have a large number buses formed by small traces, since each bus would include a large number of nodes on each bus, and since each bus would be implemented by small traces, there would be many places on the bus were a fault could occur, and any one fault would render the interconnect structure unsuitable for use in a wafer-level tester interconnect system since several ICs on each wafer would be untestable.
FIG. 2 illustrates a star bus in which the incoming signal is directly linked to each node 14 by a separate trace 16. A star bus has a number of advantages over a daisy-chain bus. Though not apparent in FIG. 2, when all traces 16 are of similar length, input signal edges will arrive at all nodes 14 at substantially the same time. A star bus distorts and attenuates signals less than a daisy-chain bus, and every node 14 sees substantially the same wave front shape. A star bus is also relatively more tolerant of open circuit faults than a daisy-chaining bus since an open circuit on any trace 16 will prevent the signal for reaching only one node 14. However even a single open circuit fault would none-the-less render an interconnect structure employing a star bus unsuitable in a wafer-testing because it would mean that one IC on each wafer would be untestable. Also since a star bus requires substantial amounts of circuit board space, it would be unsuitable as a bus in an interconnect structure for a wafer-level IC tester where a large number of buses would be concentrated into a small area.
FIG. 3 illustrates a prior art stubbed bus arrangement. The stubbed bus of FIG. 3 includes a core daisy-chain bus 20 and several daisy-chain branch buses or “stubs” 18. Each stub 18 has a proximal end connected to core bus 20 and a distal end remote from core bus 20. The traces of stubbed bus of FIG. 3 use about the same amount of PCB space as the daisy-chain bus of FIG. 1, but the stubbed bus substantially reduces variation in timing of signal edges arriving at its nodes because it reduces the variation in signal path distance the incoming signal must travel in reaching the nearest and most distant nodes 21 and 22.
The stubbed bus arrangement is often preferable over the star bus arrangement of FIG. 2 when a moderate variation in input timing at nodes 14 is acceptable because it uses less circuit board space. However the reduction in signal path delay variation over that of a daisy-chain bus is not as great as we might expect based on the decrease in signal path distances alone.
FIG. 4 is an equivalent circuit diagram of a portion of a stubbed bus interconnect system 30 distributing a test signal from a tester channel 32 to I/O pads 34 of a set of ICs 36-38. The input impedance of each IC 36-38 is modeled as a capacitor 39 (e.g., 5 pf) in series with an inductor 40 (e.g., 1.5 nH). The interconnect system is modeled as a set of 50 Ohm lossy transmission line segments 42 having series inductance (e.g., 333 nh/meter), series resistance (e.g. 0.5 Ohm/meter) and shunt capacitance (e.g., 133.3 pf/meter). An isolation resistor 46 (e.g., 1000 Ohms) is provided between bus each node on the PCB and an IC I/O pad 34. Isolation resistors 46 limit the load tester channel 32 and prevent a short circuit fault at or near any I/O pad 34 from severely attenuating a test signal passing though interconnect system 30.
Assume tester channel 32 produces a square wave test signal rising from a low logic level to a high logic level. As the test signal edge travels from tester channel 32 to all pads 34 via interconnect system 30, the interconnect system distorts the signal, and the wave front appears a little different to each IC I/O pad 34. FIG. 5 is a timing diagram illustrating the appearance of a test signal wave front 50 as it may be seen by the I/O pad 34 of the IC 36 connected to the node 21 nearest to channel 32 and the test signal wave front 52 as seen by the I/O pad 34 of the IC 38 connected to the node 22 most distant from channel 32. Wave front 50 begins its rise at time T1, and wave front 52 begins its rise a short time later at time T2. The time delay T2-T1 represents the time a signal requires to travel between the node 21 and node 22 via the most direct path. That delay is a function of the minimum signal path distance between the two nodes. If we assume that the ICs 36 and 38 recognize a state change in the test signal when its wave front rises above a threshold level (T/H) midway between the signals nominal high and low logic levels, then IC 36 will see the state change at time T3 and IC 38 will see the state change at time T4. Note that the delay between times T3 and T4 at which ICs 36 and 38 detect state changes is substantially larger than the signal path delay (T2-T1) between the two ICs.
Note also that the effective difference in signal timing at the bus nodes is thus much greater than can be accounted for by the difference in signal path lengths between the two ICs. The additional delay is caused by the difference in signal distortion. Note that wave front 50 rises more rapidly toward the T/H than wave front 52. This happens because the early portions of the wave front reaching ends of stubs are reflected back to node 21 adding to the rate at which capacitance at that node is charged prior to time T3. Since the IC 38 most distant from the test signal source 32 seeing signal 52 is near the end of a stub, the reflection has a more pronounced effect at the end of signal 52. Note the substantial overshoot of waveform 52 of FIG. 5.
The daisy-chain bus of FIG. 1 requires the IN signal to travel through 19 path segments when it travels between the first and last nodes 13 and 15. The stubbed bus of FIG. 3 requires the IN signal to travel through only 5 segments of similar length when traveling between the two most widely separated nodes 21 and 22. Thus when we abandon the daisy-chain bus of FIG. 1 in favor of the stubbed bus of FIG. 3, we might expect a 14/19ths reduction in signal timing variation. However FIG. 5 tells us that we would be disappointed; we would see a reduction in signal timing variation, but not as much as we would have expected. The stubbed bus also distorts the signals more than a daisy-chain and can produce substantial overshoot at the bus nodes. The signal distortion caused by reflections at the stubs ends is very much a function of the layout of the stub network, termination and transmission line impedances and signal frequencies. However that distortion will typically augment the variation in signal path delay.
Like a daisy-chain bus, an open circuit fault on the stubbed bus can prevent the input signal from reaching more than one node 14. Hence an open circuit fault in a stubbed bus incorporated into an interconnect system for a wafer-level integrated circuit would also render the interconnect system unsuitable for use. What is needed is a transmission line structure for conveying a signal to several PCB nodes that makes more efficient use of PCB space than a star bus, exhibits substantially less variation in signal path delay than a daisy-chain bus, provides less distortion than a daisy-chain or stubbed bus, and maintains signal integrity at all nodes in spite of an open-circuit fault.